This invention relates to a device for measuring the quantity of plasma water removed during an extracorporeal dialysis sitting.
More particularly, this device consists of a graduated container having means for connecting it to a filtering compartment of a dialyzer and other means for connecting it to the dialysis liquid circuit and is characterized in that the connection between the graduated container and the dialysis liquid circuit is made through a chamber into which extends a tube connected to the dialysis liquid circuit.
The type of dialyzer suited for the purpose is already known and is described in Italian Patent Application No. 21947 A/81. It comprises basically two bundles of hollow fibres housed in two hydraulically separated compartments. The blood of the patient subjected to dialysis flows inside each of the fibres of both bundles. The outer walls of the hollow fibres with semipermeable membranes of one of the two bundles, which is considerably bigger than the other, are in contact with the dialyzing liquid, and the compartment which contains them is connected to the dialyzing liquid circuit. The outer walls of the hollow fibres of the second bundle are not flowed over by the dialyzing liquid. The first compartment thus performs a dialyzing and purifying action while the second performs a filtering action. Depending on the type of filtration desired the type of hollow fibre of the second bundle may thus be the same as or different from that of the first bundle.
It is known that extracorporeal dialysis has the principal purpose of removing from the patients's blood a certain quantity of plasma water, urea and various metabolites.
When the quantity of plasma water removed is too great the patient is subject to various complications such as for example cramps and collapse. When it is insufficient no important effect occurs immediately but after two or three dialysis sittings the characteristic symptoms of renal insufficiency appear.
At the same time the majority of currently used dialyzers are not equipped with any device to measure the quantity of water progressively removed during the dialysis sitting. Furthermore with the same dialyzer and the same operational conditions the quantity of plasma water removed during a dialysis sitting varies from one patient to another. Under these conditions the operator who starts to subject a patient to extracorporeal dialysis regulates roughly in advance the pressure of the dialysis liquid and the venous pressure on the basis of the characteristics of the dialyzer and the quantity of water which is to be removed from the patient. After the dialysis sitting he determines the quantity of water really removed, verifying the drop in the patient's body weight.
Time by time the operator allows for the results obtained in the course of previous sittings to seek to determine the optimal operating conditions for each patient. This empirical method does not however secure satisfactory results because a certain number of variables involved escape his control. One of these variables is the hydraulic permeability of the semipermeable membranes of which the dialyzer is made and which, manufacturers being equal, has in the best of cases a variability on the order of .+-.10% but fairly often is near .+-.20%. Another variable is the patient's condition, which changes with change in diet and many other factors which escape even the most diligent operator.
To avoid these and other drawbacks known to those skilled in the art, in the course of recent years the bed scales has been used on which the patient subject to a dialysis sitting is placed and which makes it possible to determine weight loss and hence the quantity of plasma water removed progressively as the dialysis proceeds.
But neither is the bed scale free from drawbacks such as for example the very high cost thereof and the fact that during the dialysis sitting the patient usually takes liquids and/or food which prevent the operator's determining exactly the quantity of plasma water removed.
In recent years it has also been proposed to equip dialysis machines with volumetric control systems which would make it possible to determine the difference between the volume of the dialyzing liquid which enters the dialyzer and of that which issues therefrom. This system also presents the drawback of high cost; furthermore the majority of dialysis machines currently in use are not equipped with such systems and the problem of how to determine the quantity of plasma water removed with such machines remains unsolved.